It is sought more particularly here below in this document to describe problems existing in the field of seismic data acquisition for oil prospecting industry. The invention of course is not limited to this particular field of application but is of interest for any technique that has to cope with closely related or similar issues and problems.
Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure (subsurface) under the seafloor.
While this profile does not provide an accurate location for oil and gas, it suggests, to those trained in the field, the presence or absence of oil and/or gas. Thus, providing a high-resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
The operations of acquiring seismic data on site conventionally use networks of seismic sensors, like accelerometers, geophones or hydrophones. We consider below the context of seismic data acquisition in a marine environment, in which the seismic sensors are hydrophones. The hydrophones are distributed along cables in order to form linear acoustic antennas (also referred to as “streamers” or “seismic streamers”). As shown in FIG. 1, a seismic vessel 21 tows the network of seismic streamers 20a to 20e. The hydrophones are referenced 16 in FIG. 2, which illustrates in detail the block referenced C in FIG. 1 (i.e. a portion of the streamer referenced 20a).
The seismic method is based on an analysis of reflected seismic waves. Thus, to collect geophysical data in a marine environment, one or more submerged seismic sources are activated in order to propagate omnidirectional seismic wave trains. The pressure wave generated by the seismic source(s) passes through the column of water and propagates through the different layers of the seabed. The reflected seismic waves (i.e. reflected acoustic signals) are then detected by the hydrophones distributed over the length of the seismic streamers. These acoustic signals are processed and retransmitted by telemetry from the seismic streamers to the operator station situated on the seismic vessel, where the processing of the raw data is carried out (in an alternative solution, the seismic acoustic signals are stored for a later processing).
During seismic surveys, it is important to precisely locate the streamers in particular for monitoring the position of the hydrophones (distributed along the seismic streamers) in order to obtain a satisfactory precision of the image of the seabed in the exploration zone; for detecting the movements of the streamers with respect to one another (the streamers are often subjected to various external natural constraints of variable magnitude, such as the wind, waves, currents); and for monitoring the navigation of streamers, in particular in a situation of bypassing an obstacle (such as an oil barge).
Control of the positions of streamers lies in the implementation of navigation control devices (also referred to as “birds”) (white squares referenced 10 in FIG. 1). They are installed at regular intervals (every 300 meters for example) along the seismic streamers.
Examples of birds are disclosed in U.S. Pat. No. 9,475,553, the entire content of which is incorporated herein by reference.
The function of birds is to guide the streamers between themselves. In other words, the birds 10 are used to control the depth as well as the lateral position of the streamers.
For this purpose, and as illustrated in FIG. 2, each bird 10 includes a body 11 equipped with motorized pivoting wings 12 (or more generally means of mechanical moving) making it possible to modify the position of the streamers laterally between them (this is referred to a horizontal driving) and drive the streamers in immersion (this is referred to a vertical driving).
To carry out the localization of the seismic streamers (allowing a precise horizontal driving of the streamers by the birds), acoustic nodes are distributed along the streamers. These acoustic nodes are represented by hatched squares, referenced 14, in FIGS. 1 and 2. As shown in FIG. 1, some acoustic nodes 14 of the network are integrated in a bird 10 (case of FIG. 2), and others are not.
Birds must be reliable and robust to keep depth and lateral control on the equipment constituted by the birds themselves and by the instrumented cables of the streamers.
As any electromechanical equipment, a bird can face internal failures that can lead to a loss of control. Furthermore, a bird and a sub-sea equipment are exposed to harsh environment, such as ice, fishing nets or debris, that can damage it, the bird being then likely to breaking down, not responding to the orders, or loosing control.
In case of loss of control or other failure of a bird, the following damages can occur. Sensors can be affected by an excessive depth of the streamer. There can be a mechanical deformation on cable and/or birds due to excessive depth, overload, excessive torsion or tangle between the cables.
Other external failures may occur such as the failure of the vessel or of the monitoring system, leading to damages of the sensors and/or birds and/or streamers. Indeed, as already said, the streamers and birds are exposed to a harsh environment, and a portion of streamer may be cut, for example by a shark, an excessive tension or an excessive twisting or by hanging to an anchor.
A known solution is to add additional equipment, commonly called “retriever”, to the marine equipment, such as the bird, that is to be retrieved.
A retriever system is for example known from U.S. Pat. No. 5,404,339. The retriever system comprises a hollow body that includes a big bag and a bottle of gas for blowing the bag when predetermined excessive depth is reached by the retriever so that the big bag exits the hollow body.
However, known retriever systems have drawbacks. In particular, it happens that the big bag be damaged when exiting the hollow body. Furthermore, known retriever systems are cumbersome causing noise on surrounding sensors. Also, it requires significant space for storage on board a vessel. Finally, it is heavy to handle during operations on a vessel deck.
Thus, there is a need to provide retriever systems and methods for retrieving marine equipment that enable to overcome at least part of the drawbacks of the known retriever systems.